This invention relates to a method and apparatus for recovering clock information from a received digital signal and, more particularly, to a relatively simple yet accurate technique for recovering inherent clock information from a digital signal in which binary information is represented by signal level transitions which are present on a periodic basis, such as a transition at a mid-location of a bit interval. The present invention further relates to a technique by which the digital signal is re-synchronized; and the recovered clock information is used to detect loss of synchronism and to control the resynchronizing operation.
Various data communication techniques call for the resynchronizing of digital information which is transmitted from one location and received at another, remote location. Such resynchronizing, or re-clocking, is intended merely to restore the original transmission rate which, because of various factors, might be subjected to time delays, phase shifts, and other deviations. This need for resynchronizing a digital signal is most notable in long distance digital transmission systems, systems in which the transmission medium might change, and so-called network transmission systems (such as local area networks) in which a common transmission bus is connected to several stations. In systems of the aforementioned type, it often is desirable to re-clock the digital signal to restore its original transmission rate. In other systems, however, it may be desirable to re-clock the digital signal to a new transmission rate which may be greater (or less than) the original.
A change in the transmission medium in a digital signal communication system is encountered when fiber optic media are used. The advantages of using a fiber optic transmission link in place of conventional conductors (such as coaxial cables) are sufficiently known and understood as to require no further description herein. Since a fiber optic transmission link interconnects electrical transmitting and receiving devices which operate upon conventional electrical signals as opposed to light, a conversion of electrical parameters to optical parameters, and vice versa is needed. Fiber optic links utilize electro-optic repeaters, often referred to simply as modems (or modulator-demodulator devices) in which re-clocking is carried out. Typically, a re-clocking operation is employed on the converted electrical signal at a receiving station, or repeater, at which a fiber optic transmission link terminates. Then, the re-clocked digital signal is reconverted back to optical form and transmitted to the next receiver or repeater.
Other examples of a change in transmission medium which suggests the need for a repeater or modem to carry out a re-clocking operation include radio wave transmission converted to transmission over electrical conductors; a wave guide-to-coaxial cable interface; and the like.
To enhance the re-clocking of digital signals, such as the re-clocking found in repeaters, modems, and the like, as mentioned above, various so-called "self-clocking" codes have been proposed. Typically, these codes are used to represent binary signals, or bits, by providing signal level transitions at or in the vicinity of the mid-location of each bit interval of the digital signal. One of these codes, known as the Manchester code, represents a binary "1" by a positive-going transition at the mid-location of a bit interval, and a binary "0" is represented by a negative-going transition. In another code, known as a differential code, the binary "0" is represented by a signal level transition at the mid-location of a bit interval which is of opposite polarity to the transition which immediately preceded it. A binary "1" is represented in this code as a signal level transition of the same polarity as that which just preceded it. These codes are known as self-clocking codes because the signal level transitions represent not only binary information but also the clock rate at which that binary information is encoded (or transmitted). Other self-clocking codes include the phase coherent code, the alternate mark inversion (AMI) code and the split phase code.
To receive a self-clocking encoded digital signal accurately, without ambiguity as to the nature of each bit included therein, it is important to recover the clock information which inherently is encoded in that signal. Of course, the recovery of such clock information is needed to resynchronize the digital signal for re-transmission. However, techniques which have been used heretofore to recover that clock information are subjected to ambiguity. This is because of apparent changes in the clock rate (or repetition rate) of many self-clocking encoded signals. For example, in a Manchester encoded signal, successive bits of the same polarity (e.g. two or more successive binary "0"s or two or more successive binary "1"s) exhibit twice the repetition rate of alternating "1"s and "0"s. Because of this characteristic, it is not enough merely to detect signal level transitions in the received digital signal as a representation of the clock information included therein. In the case of repeated bits of the same polarity, a signal level transition will occur both at the mid-location of a bit interval as well as at the end (or beginning) of that interval. Hence, if clock pulses are generated solely in response to each signal level transition in the digital signal, an "extra" clock pulse will be produced for each bit interval when repeated bits of the same polarity are present.
When using a common bus coupled to several stations (each of which may include a repeater or a modem), control circuitry is provided to prevent ambiguity in the event that two (or more) stations wish to seize the bus at precisely the same time. Such control techniques prevent so-called "collision" when more than one station transmits simultaneously to the bus. Although multiplexing arrangements may be used to interleave signals from various stations, simultaneous transmissions of a bit at the very same time are avoided. However, collisions may occur; and it is important to prevent the re-transmission of errors and distortions due to such collisions. It is important, therefore, to detect collisions so as to prevent re-transmitted distortions. In this regard, it has been found that a collision will cause the apparent repetition rate of a received digital signal to differ significantly from the expected repetition rate.